class BuildDemo

class BuildDemo extends FormBase {
    
    /**
     * Counter keeping track of the sequence of method invocation.
     *
     * @var int
     */
    protected static $sequenceCounter = 0;
    
    /**
     * {@inheritdoc}
     */
    public function __construct() {
        $this->displayMethodInvocation('__construct');
    }
    
    /**
     * Update form processing information.
     *
     * Display the method being called and it's sequence in the form
     * processing.
     *
     * @param string $method_name
     *   The method being invoked.
     */
    private function displayMethodInvocation($method_name) {
        self::$sequenceCounter++;
        $this->messenger()
            ->addMessage(self::$sequenceCounter . ". {$method_name}");
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['description'] = [
            '#type' => 'item',
            '#markup' => $this->t('Demonstrates how submit, rebuild, form-rebuild and #ajax submit work.'),
        ];
        // Simple checkbox for ajax orders.
        $form['change'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Change Me'),
            '#ajax' => [
                'callback' => '::ajaxSubmit',
                'wrapper' => 'message-wrapper',
            ],
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        // Add a submit button that handles the submission of the form.
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => 'Submit',
        ];
        // Add button handlers.
        $form['actions']['button'] = [
            '#type' => 'button',
            '#value' => 'Rebuild',
        ];
        $form['actions']['rebuild'] = [
            '#type' => 'submit',
            '#value' => 'Submit Rebuild',
            '#submit' => [
                '::rebuildFormSubmit',
            ],
        ];
        $form['actions']['ajaxsubmit'] = [
            '#type' => 'submit',
            '#value' => 'Ajax Submit',
            '#ajax' => [
                'callback' => '::ajaxSubmit',
                'wrapper' => 'message-wrapper',
            ],
        ];
        $form['messages'] = [
            '#type' => 'container',
            '#attributes' => [
                'id' => 'message-wrapper',
            ],
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        $this->displayMethodInvocation('getFormId');
        return 'form_api_example_build_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        $this->displayMethodInvocation('validateForm');
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->displayMethodInvocation('submitForm');
    }
    
    /**
     * Implements ajax submit callback.
     *
     * @param array $form
     *   Form render array.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   Current state of the form.
     */
    public function ajaxSubmit(array &$form, FormStateInterface $form_state) {
        $this->displayMethodInvocation('ajaxSubmit');
        $form['messages']['status'] = [
            '#type' => 'status_messages',
        ];
        return $form['messages'];
    }
    
    /**
     * Implements submit callback for Rebuild button.
     *
     * @param array $form
     *   Form render array.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   Current state of the form.
     */
    public function rebuildFormSubmit(array &$form, FormStateInterface $form_state) {
        $this->displayMethodInvocation('rebuildFormSubmit');
        $form_state->setRebuild(TRUE);
    }

}